1. Field of the Invention
The present invention is generally related to the sensing and transmission of acoustical signals through the use of fiber optics. More particularly, the present invention is directed to the use of a unidirectional coupler in conjunction with single optical fibers to produce an effective acoustical sensor apparatus.
2. Description of the Prior Art
Fiber optics have been suggested for use in telephone systems, and in fact prototypes are now in use. It has been projected, based on demonstrations, that most new telephone lines in the 21st century will be optical fibers. They have been shown to be useful in short-range telephone applications, such as within a telephone transceiver. One system has demonstrated delivery of both the power for the electronic circuitry in the telephone transceiver and transmission of audio signals by optical fibers. The suggested systems, however, still utilize opto-electronic transducers to convert light signals to electronic signals and vice versa. In addition, electroacoustic transducers are used to convert electronic signals to acoustical signals and vice versa. Direct conversion of the acoustic signals to an optical signal has been explored but with only limited success in the past.
It has been recently discovered that optical transmission properties for fibers are pressure sensitive and that when a sound wave is transmitted through a fluid medium, such as air or water, to a coiled optical fiber a significant modulation of the light beam in the fiber results. However, the interaction is weak and a large bulky coil of fiber is required. Other methods, such as vibrating mirrors positioned at the end of a fiber, have not been found to be practical in telephone receiver use.
One system utilizing optical transducers, lenses and mirrors, without using optical fibers is disclosed in U.S. Pat. No. 3,175,088 to Herriott. Herriott discloses the basic concept of utilization of reflection of light by an acoustically sensitive surface to produce an acoustical signal. Fulenwider in U.S. Pat. No. 4,071,753, on the other hand, utilizes various means for adjusting the alignment of two fibers, positioned in an end-to-end relationship, to vary the light transmitted from fiber to fiber. Fulenwider also discloses a pressure sensitive membrane with a reflective surface.
Numerous patents suggest the use of plural fibers joined in a bundle, some of which are utilized to transmit an optical signal to an acoustically sensitive surface and some of which are used to return it to a transducer. Examples include U.S. Pat. Nos. 3,831,137 of Cuomo; 3,903,496 of Stimler; 3,940,608 of Kissinger; and 4,158,310 of Ho.
Strack in U.S. Pat. No. 3,580,082 discloses a pressure transducer that utilizes a single source fiber. The transducer is manufactured so that the source fiber is surrounded by plural, smaller, receiver fibers. The signal is transmitted through the source fiber and reflected off an acoustically sensitive membrane to the surrounding small transmission fibers to generate the resulting modulated signal.
All of these systems require a minimum of two fibers, one for transmission and one for reception, in order to be able to function. Thus the alignment between the two fibers is critical so that the signal modulation obtained will be maximized.